Antimicrobial resistance in swine production

Animal Health Research Reviews 9(2); 135–148
Antimicrobial resistance in swine production
Frank M. Aarestrup1*, C. Oliver Duran2 and David G. S. Burch31National Food Institute, Technical University of Denmark, Bulowsvej 27, DK-1790 Copenhagen V,Denmark2Moss Veterinary Partners IDA Estate, Monread Road, Naas, Co. Kildare, Ireland3Octagon Services Ltd, Old Windsor, Berkshire, UK
Received 15 September 2008; Accepted 22 September 2008; First published online 5 November 2008
AbstractLarge amounts of antimicrobial agents are still being used in modern swine production in manycountries around the world. This facilitates the emergence and development of antimicrobialresistance. Bacteria causing infections in swine have in several cases acquired resistance to anumber of the agents most commonly used for treatment, making it difficult to predict theefficacy of different antimicrobial agents without prior susceptibility testing. This review givesan overview of recent susceptibility data from different parts of the world and discusses theimportance of the development of resistance not only in the treatment of infections in swinebut also taking into account the human health implications of antimicrobial resistance.
Keywords: antimicrobial resistance, swine, treatment, Escherichia coli, Brachyspira, Actino-bacillus pleuropneumoniae, Streptococcus suis, Staphylococcus hyicus, Mycoplasma, Lawsonia,Clostridium, Pasteurella
Emphasis is being placed on the need to target the use ofantimicrobials towards the specific pathogen and only
Pork is one of the most commonly consumed food
commodities globally. The production of pork ranges
Antimicrobial resistance has emerged among bacteria
from the highly intensive, volume and efficiency driven
causing infections in swine in several countries. In some
systems, to ‘backyard’ production with one or a few pigs
cases this makes empiric therapy difficult, whereas it is
per family. In both cases, diseases can greatly affect the
still possible to predict the susceptibility of other patho-
cost of production. Thus, especially in intensive and
gens. This review gives an overview of the occurrence of
large-scale production, the routine use of antimicrobials
resistance among the most common swine pathogens, the
has become an integrated part of the production system.
trends we currently observe and a discussion of the trends
Antimicrobial agents have not only been used for
treatment of clinically ill pigs, but also as part of theroutine management for prophylaxis and even growthpromotion. Sale of antimicrobials for use in swineproduction are reported to be worth an estimated 1.7
Most common pathogens causing infection in swine
billion dollars, equal to 34% of the global animal healthantimicrobial market followed by poultry (33%) and cattle
Precise estimates of the prevalence of porcine infections
and the consumption of antimicrobial agents used to treat
Because of the human (and animal) health aspects
or control those infections are difficult to obtain, even
associated with the development of antibiotic resistance,
though most veterinarians have a fairly good idea of the
as a consequence of this widespread use, medication
incidence in farms under their care. However, only a very
with antimicrobials has come under increased scrutiny.
limited number of countries report data on antimicrobialusage and/or prevalence of infections. In Denmark, amonitoring programme for antimicrobial use has been
*Corresponding author. E-mail: [email protected]
in place since 2000 ( Jensen et al., 2004), which monitors
Mill. animal daily dosages
Fig. 1. ADDs used for treatment of weaners, sows and slaughter pigs in Denmark in 2007.
drug use at the farm and diagnostic level and also
data are, however, available from diagnostic laboratories.
calculates the number of animal daily dosages (ADDs)
Figure 2 shows the percentages of diagnosis of diseases of
used for different age groups and by diagnosis. The usage
the digestive system at Veterinary Laboratories Agency,
of antimicrobial agents for the different age groups
(http://www.defra.gov.uk/vla/reports/docs/rep_
and the different antimicrobial agents are calculated into
vida_pigs99_06.pdf). The calculation is based on a total
ADDs to get a better comparison because the activity of
of 3188 diagnoses in the period 1999–2006. In 1999,
the various antimicrobial agents differ substantially and
E. coli accounted for almost 50% of all diagnoses of
the amount necessary to treat e.g. a sow of 150 kg is
diseases in the digestive system. This has, however,
higher than the amount needed to treat a pig of 30 kg.
changed considerably and diseases related to Lawsonia
The number of ADDs used for treatment of different
and Brachyspira seem now to be more important.
infections in sows/piglets, weaners and slaughter pigs in
Whether this is due to a real change in the importance
Denmark in 2007 is given in Fig. 1. It is very clear that the
of the disease or changes in the diagnostic abilities is
majority of treatments are for gastrointestinal infections
however, unknown. The distribution of the most impor-
in weaners (>170 million ADDs in 2007). It is noteworthy
tant species among respiratory infections is given in Fig. 3.
that this implies that the approximately 25 million pigs
In England, the most common bacterium is seemingly
produced in Denmark each year on average are treated
Pasteurella multocida and is followed by Actinobacillus
for approximately seven days during their weaning period.
pleuropneumoniae. Conversely, in the USA based on
General infections are almost exclusively in weaners,
submissions to the Iowa State University Veterinary
whereas respiratory infections are more commonly in
Diagnostic Laboratory during 2006 (over 28,000 cases)
slaughter pigs and not surprisingly treatment of repro-
the frequency of A. pleuropneumoniae and Mycoplasma
ductive and urogenital organs and the udder are in the
hyopneumoniae diagnosis has declined in the last 5 years
(Madson, 2008) (Fig. 4). Other respiratory bacterial
The treatment incidence does, however, not give any
pathogens, like S. suis, Haemophilus parasuis and
information on the causative agent. The common bacterial
Actinobacillus suis have been isolated more frequently
infections are summarized in Table 1. They are divided
by this diagnostic laboratory. These differences probably
into primarily enteric, respiratory and other infections.
reflect the different nature of the production systems,
There is some overlap, since e.g. Escherichia coli can
eradication programs for major swine diseases and the
be both septicemic and enteric, especially in neonatal
influence of immunosuppressive viruses.
piglets, and Streptococcus suis can be isolated from the
The number of bacterial and viral diagnoses during the
respiratory tract as well as the central nervous system.
first 6 months of 2007 at the National Veterinary Institute
Precise estimates of the prevalence of the various
in Denmark is shown in Fig. 5 (http://www.dfvf.dk/
bacterial diseases are difficult to obtain. Practising
Default.aspx?ID=21768). The most common swine patho-
veterinarians often do not collect disease incidence data
gen diagnosed is A. pleuropneumoniae, followed by
in a systematic way and data obtained from diagnostic
E. coli and S. suis. Thus some clear differences do exist
laboratories may be biased by the fact that veterinarians
between Denmark and England, but the general pattern
mainly submit samples from difficult clinical cases. Some
seems to be that the enteric pathogens Brachyspira,
Antimicrobial resistance in swine production
Table 1. Common bacterial infections and diseases in the pig
Choleraesuis – septicemia diarrhea, death
Porcine proliferative enteropathy (ileitis)
Growers and finishers, 6–26 weeksAll ages in primary breakdown
Gla¨sser’s disease (arthritis, pericarditis,
time systemic-acting antimicrobials can be used effec-
A. pleuropneumoniae and P. multocida, and the more
tively. Piglet scours are usually less severe but almost all
systemic pathogen S. suis are the most common and
pigs suffer some form of post-weaning check. Diarrhea
starts 4–5 days after weaning and can lead to dehydration
Despite the lack of monitoring data from different
and mortality in severe cases. The severity can be
countries we have attempted to depict the basic patterns
mitigated by a good stable temperature and clean
of infections in Fig. 6, divided into enteric infections,
environment, weaning at 4 weeks of age or older, not
respiratory infections and general infections, especially
mixing litters, carefully formulated diets and by the
focusing on the first 24 weeks of the pig’s life, since this is
addition of therapeutic levels of zinc oxide in the diet.
Once over this period, there are usually few problemswith E. coli, except for cases of bowel edema, associatedwith verocytotoxic strains and sometimes after moving
Antimicrobial resistance among the major pathogens
into a new, colder house. The susceptibility pattern ofE. coli in different countries is shown in Table 2. A very
high frequency of resistance is found in some countriesand the antimicrobial susceptibility of E. coli is difficult
to predict, which means that the final choice of antibiotic
E. coli primarily affect the younger pig. Neonatal scours
has to be based on knowledge of the local situation
can be severe and the piglets can die of septicemia. At this
and preferably susceptibility testing. E. coli are also
Percent of diagnosis
Fig. 2. Trends in percentage of selected bacterial pathogens from digestive diseases in pigs from the Veterinary LaboratoriesAgency in the period 1999–2006.
PRRSV and PCV2 infections. The antimicrobial suscept-
ibility among Salmonella spp. is intensively surveyed
because of the zoonotic importance of this bacterium. In
general antimicrobial treatment is not recommended in
animals because this might lead to resistance develop-ment and thus, human health problems. Specific data onthe occurrence of resistance in S. Choleraesuis are limited.
However, a high frequency of resistance has been
reported from the US, Taiwan and Japan (Chang et al.,2002a, b; Esaki et al., 2004; Zhao et al., 2007) makingempiric treatment difficult. Recent data from the US in-
dicate widespread S. Choleraesuis resistance to ampicillin,
tetracyclines and sulfonamides, but susceptibility toaminoglycosides, trimethoprim/sulfamethoxazole, fluoro-
Fig. 3. Percentages of respiratory diagnoses from the
quinolones and cephalosporins (Madson, 2008). Thus, as
Veterinary Laboratories Agency in the period 1999–2006
for E. coli treatment has to be based on local experience
attributed to various bacterial species. The total number of
Clostridium spp.Clostridium perfringens type C is mainly associated with
considered generally susceptible to polymyxins. These
per-acute hemorrhagic and necrotic enteritis in young
substances are not always included in routine suscept-
piglets, which can be fatal. The disease is not very
ibility testing, but could be a reliable drug for treatment of
common, and is controlled mainly by sow vaccination.
Infections in older piglets and growing pigs is less severeand also in growing pigs and usually associated with
type A strains. Clostridium difficile have recently emerged
Many strains of Salmonella have a low pathogenicity in
as a cause of infections in pigs (Songer, 2004). The
pigs and are more of a concern for zoonotic transmission.
infections are associated with mild diarrhea and ill thrift in
However, S. Choleraesuis is highly pathogenic in pigs and
piglets and like in human medicine appears related to
is associated with acute outbreaks of diarrhea, septicemia
the use of antimicrobial agents, mainly cephalosporins.
and death especially in finishing pigs. This serovar is
There are only very few reports on antimicrobial sus-
rarely isolated in Europe but is commonly reported in the
ceptibility from C. perfringens or C. difficile from pigs.
US and Asia. S. Typhimurium can cause diarrhea, wasting,
In the 1970s in Wisconsin, USA, Rood et al. (1978)
septicaemia and death. The incidence increases with
examined 258 C. perfringens isolates from six pig farms
Antimicrobial resistance in swine production
Percent of diagnoses
1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Fig. 4. Percentages of 50,316 positive respiratory disease diagnoses from diagnostic samples from swine during 1994–2005that were attributed to various pathogens. The data are from the Department of Veterinary Diagnostic and production AnimalMedicine at Iowa State University, Ames, Iowa (Madson, 2008).No. diagnosis
Fig. 5. Number of positive diagnoses from diagnostic samples from swine at the National Veterinary Institute in Denmarkduring the first 6 months of 2007.
routinely using antibiotics in feed and 240 isolates from
without the availability of nitroimidazoles like metroni-
five farms that did not. They found 78% tetracycline
resistance and 23% macrolide resistance among isolatesfrom the antibiotic using farms in comparison to 25%
tetracycline resistance and 0.8% macrolide resistance
L. intracellularis is a relatively ubiquitous organism
among the farms not routinely using in feed antibiotics.
on pig farms. Various surveys have showed that 80–95%
Post and Songer (2004) examined the susceptibility of
of farms are infected. It is commonly associated with
80 C. difficile isolated from piglets with diarrhea. They
diarrhea in growing pigs and primarily affects the ileum
did not report full range MICs or percent resistance. The
although the organism can be found in caecal and colonic
data suggest that all C. difficile are resistant to bacitracin
epithelial cells. Susceptibility testing is difficult because
and ceftiofur and that some isolates have acquired
the organism can only be grown in cell cultures. Based on
resistance to macrolides, tetracycline, tiamulin and
clinical experience tetracyclines, tiamulin, valnemulin,
virginiamycin. Penicillins may be used for treatment of
tylvalosin and tylosin seem to be effective in controlling
C. perfringens infections, whereas treatment of infections
the disease. However, there are currently limited data
with C. difficile, as in human medicine, might be difficult,
on the development of resistance or the establishment of
Clinical incidence/risk (%)Clinical incidence/risk (%)Age (weeks)Clinical incidence/risk (%)Age (weeks)
Fig. 6. Disease patterns among pigs. (A) enteric diseases; (B) respiratory diseases and (C) systemic diseases.
breakpoints for L. intracellularis but Wattanaphansek
Canada. Recent reports from the US suggest that
et al. (2007) demonstrated high intracellular MICs to
B. hyodysenteriae may be a re-emerging pathogen
chlortetracycline, lincomycin and tylosin, particularly in
(Duhamel, 2008). It causes severe diarrhea, commonly
some US isolates suggesting that resistance can occur. In
with mucus and blood and leads to rapid wasting and also
comparison, carbadox, tiamulin and valnemulin all had
dehydration and death. Brachyspira pilosicoli is wide-
spread as a low-grade cause of mucoid diarrhea, eitheralone or in mixed infections and can be associated
with PCV2 infections. There is only a limited number of
Brachyspira hyodysenteriae, the cause of swine dysen-
antimicrobial agents available for treatment of infectious
tery, is a recurring severe problem in many countries in
caused by Brachyspira spp. in pigs. The slow develop-
Europe, but seemingly less important in the US and
ment of immunity in infected pigs, the persistence of
Table 2. Occurrence of antimicrobial resistance among E. coli isolated from infections in swine in different countries
Percentage of E. coli that were resistant
Hendriksen Boerlin Hendriksen Hendriksen Hendriksen Hendriksen Schro¨er
Harada Hendriksen Hendriksen Hendriksen Hendriksen Hendriksen Hendriksen Hendriksen
Table 3. Antimicrobial resistance among B. hyodysenteriae and B. pilosicoli in various countries
Percentage of the isolates that were resistant
B. hyodysenteriae in the environment and the under-
have been almost pan-susceptible. However, as can be
dosing of in feed medication in pigs with a reduced
seen from Table 4, this is no longer the case, since
appetite have undermined the effectiveness of available
resistance to the beta-lactam antibiotics has emerged.
antimicrobial agents. Resistance to macrolides (tylosin)
Resistance to tetracycline and other antimicrobials has
and lincosamides seems to be very high in many countries
also emerged, but most isolates seem still susceptible to
and the most active agent are the pleuromutilins tiamulin
fluoroquinolones, ceftiofur and florfenicol.
and valnemulin, where most isolates seem to besusceptible (Table 3). However, recently isolates with
reduced susceptibility to pleuromutilins have emerged,
Atrophic rhinitis is caused by a mixed infection of
both among B. hyodysenteriae (Lobova et al., 2004;
Bordetella bronchiseptica and P. multocida and usually
Rohde et al., 2004) and B. pilosicoli (Pringle et al., 2006).
starts in young pigs from 7 to 10 days of age. Clinically
It is essential that detailed susceptibility testing of
there is sneezing and the bacteria colonize the nasal
B. hyodysenteriae be carried out in all Swine Dysentery
mucosa and the toxins, usually from Type D P. multocida,
cases. Short antibiotic courses at effective doses and
cause the destruction of the turbinate bones. The main
enhanced pen and farm sanitation must be used at all
nasal bones may grow unevenly causing twisting and
times. Eradication of B. hyodysenteriae from farms can be
foreshortening as the pig grows. The disease can be
readily achieved and would be recommended to reduce
effectively controlled by vaccination of the sows and
the risk of antimicrobial resistance and inability to control
antimicrobial therapy is rarely needed. Until now a low
clinical outbreaks of swine dysentery.
frequency of acquired resistance to ampicillin, chloram-phenicol, tetracycline and TMP-sulfonamides has beenreported among B. bronchiseptica (Kadlec et al., 2004).
P. multocida from pigs is generally susceptible topenicillins, ceftiofur, gentamicin, macrolides, fluoroqui-
Many porcine bacteria can be found in the respiratory
nolones, tetracyclines, trimethoprim-sulfonamides and
tract, but also can be found systemically and cause
florfenicol, even though some resistance seems to have
meningitis, arthritis, pleurisy, pericarditis and peritonitis,
emerged, whereas more resistance is observed towards
streptomycin (Yoshimura et al., 2001; Lizarazo et al., 2006;Wallmann, 2006).
A. pleuropneumoniaeA. pleuropneumoniae can cause primary acute necrotiz-
ing pneumonia on its own or in combination with
M. hyopneumoniae, the cause of enzootic pneumonia, is
M. hyopneumoniae. Some serotypes given in artificial
endemic throughout the world, with most herds being
infection studies can cause death within 24 h, due to the
infected. On its own, it causes a relatively mild disease,
toxic shock produced by its exotoxins. Treatment has
the damage it does to the cilia lining the respiratory tract
traditionally been using penicillins where the isolates
and the immuno-suppressive effect it has in the lung
Antimicrobial resistance in swine production
permits a number of bacteria, especially P. multocida, to
colonize the lung and cause broncho-pneumonia. Myco-plasma hyosynoviae is the cause of mycoplasmal arthritis.
Worldwide there have been very few reports on the
antimicrobial susceptibility of Mycoplasma from pigs.
Some isolates seems to have acquired resistance to
tetracyclines, fluoroquinolones and macrolides, whereasresistance to tiamulin has not been reported (Aarestrup
and Kempf, 2006). Thus, based on the available suscept-
ibility data, tiamulin seem to be the best choice forempiric treatment. More data on clinical efficacy does
however, seem to be needed. However, for M. hyopneu-moniae, vaccines have become the main method of
control with some countries vaccinating over 50% of thenational growing herd.
S. suisS. suis is widespread in many herds but do not always
cause clinical problems. A large number of different
serovars can be found, but the most important seem to be
serotypes 2 and 7, with S. suis type 2 as the most common
associated with meningitis in weaner and grower pigs.Susceptibility data are presented in Table 5. Some
variations in the susceptibility pattern can be observed
between different surveys. A frequent occurrence of
resistance to macrolides and tetracycline is found in most
studies. In most reports a low frequency of resistance topenicillins is reported. However, resistance to this group
of antibiotics seems to be emerging in some countries,
potentially making treatment of S. suis difficult, since
penicillins typically have been the drug of choice against
H. parasuis causes infections in weaners and growers,especially polyserositis (Gla¨sser’s disease). In somecountries H. parasuis is almost pan-susceptible to all
tested antimicrobial agents, whereas high frequencies of
resistance seemingly have emerged in other countries
(Table 6). Thus, compared to just a few years ago, when
penicillins could almost always be expected to be
effective against H. parasuis, treatment now has to be
based on local knowledge and continuous monitoring.
Erysipelothrix rhusiopathiaeE. rhusiopathiae can cause arthritis and valvular endo-
carditis following a septicemic episode, but is more
commonly associated with the classic diamond-shaped
skin lesions. In recent years, following partial depopula-
tions and clean ups in herds to reduce respiratory disease,peracute outbreaks of erysipelas have been morecommonly seen. There are a number of effective vaccines
available and treatment is rarely needed. E. rhusiopathiae
is susceptible to penicillins, which is effective in treat-
ment. There is some development of resistance to
tetracycline, streptomycin and macrolides (Yamamoto
Staphylococcus hyicusGreasy pig disease, caused by S. hyicus, is a sporadic
disease affecting young pigs from 7 to 35 days of age. It isthought that the infection gets into the skin following
trauma from fighting, rough concrete sharp protrusions,
etc., which enables the organism to penetrate. It causes a
generalized dermatitis and an excessive secretion of
sebum and exudates, which causes a greasy dark cover-ing to the skin. A number of countries have reported data
on antimicrobial susceptibility among S. hyicus (Table 7).
In general a high frequency of resistance is found to
macrolides, tetracycline, sulfonamides and streptomycin,
whereas the isolates still seem to be susceptible to flor-
fenicol, fluoroquinolones and gentamicin. It is difficult
to predict the susceptibility of S. hyicus and treatment has
to be performed according to knowledge of the specific
farms and routine testing of the pathogen.
Staphylococcus aureusS. aureus is an important opportunistic pathogen for most
animal species and causes a variety of different infec-
tions including skin infections, septicemia, osteomyelitis,
arthritis and pneumonia. Recently, a special methicillin-
resistant S. aureus (MRSA) isolate (CC398) has emerged
among production animals, primarily swine in many
countries (Wulf and Voss, 2008). This type has gained
intensive attention because it might colonize healthy
swine and spread to humans through direct contact,
such as farmers and veterinarians. However, S. aureus ispotentially an important pathogen for swine and also
MRSA of CC398 has been observed as a cause of
infections in pigs (van Duijkeren et al., 2007). There isonly limited information on the susceptibility of S. aureus
from infections in pigs. Unpublished data from Denmark
and data from The Netherlands (van der Wolf et al., 2008)
suggest that resistance to macrolides, streptomycin and
tetracycline is frequent, whereas the isolates are in
general susceptible to TMP-sulfonamides and fluoroqui-
nolones. It is, however, difficult to predict the suscept-
ibility and the potentially continued emergence of MRSA,
which might not only have implications for human health,
but might also make it more difficult to treat infections in
General principles for disease control in
Disease control is not only about using medicines.
Frequently, what has gone wrong is the production
system; hence the challenge is to correct the underlying
management problems. Post-weaning diarrhea is the
classic example. If the temperature of the weaning
accommodation is kept high and constant and drafts are
avoided, there is normally little trouble. The ‘correct’
environment is very important to the pig and disease
Antimicrobial resistance in swine production
Table 6. Antimicrobial susceptibility of H. parasuis
Percentage of isolates that were resistant
Table 7. Occurrence of antimicrobial resistance (%) in S. hyicus from different countries (from Aarestrup and Schwarz 2006)
Percentage of isolates that were found resistant
1Both healthy and diseased animals.–, Not tested.
prevention. In general, two approaches can be used,
avoid the infectious agents and avoid the clinical disease.More details can be read in Burch et al. (2008). Avoiding
Small closed breeding finisher herds, which are family
the infectious agent can be achieved by starting up a herd
owned, often do better than farms where pigs are looked
free of infectious diseases or by carrying out depopulation
after by employees. In addition, avoid mixing pigs of
and repopulation with clean stock. Once established, it is
different ages and/or immunity status, such as those
crucial to avoid buying animals from farms with diseases
coming from different farms. Avoid stress by using
as well as ensuring strict biosecurity when entering the
production systems based on reduced moving and mixing
farm. Avoiding the clinical disease might be more difficult.
of pigs. The benefit of raising pigs segregated by the
The production systems in every pig-producing region of
parity of the sows is also well established since this
the world and almost each farm are different and have
reduces the pathogen transmission between groups of
their own problems. However, even though this is a
pigs with similar immune status. Another important man-
complex situation, some basic principles still apply. Thus,
agement point is the age at weaning. One of the main
there are three key areas for avoiding clinical disease,
problems with enteric diseases in pigs comes from
which need to be addressed: herd management, pig
weaning the piglets before their immune system is
housing and environment, and immunity.
consequences should also be taken into account.Recently, the World Health Organization has developed
Particularly for respiratory diseases, a reduction in pigs
a list of critically important antibiotics for human health
per airspace has resulted in less severe infections,
(WHO, 2007) and it is recommended that the use of these
although some of these benefits can also be reached
agents in food animal production be limited as much as
with correct ventilation and management. Increased pig
density in pens or barn has also been linked withincreased stress and disease transmission resulting inhigher mortality and reduced growth.
As can be seen from the examples provided in this reviewthe occurrence of antimicrobial resistance varies greatly
between countries and even regions and individual herds.Thus, the final choice of empiric treatment has to be
Understanding the development of immunity in a herd or
based on the local situation. This requires regular
group of pigs will allow better control of diseases on farm.
susceptibility testing of the pathogens involved in the
Excellent colostrum intake in the first 6 h of life will
diseases to guide the veterinarians. Especially enteric
ensure good protection against many piglet infections.
bacteria, such as E. coli, have in some cases developed
Grouping of pigs to ensure a common immune status will
resistance to all available antimicrobial agents and the
reduce the susceptible population and reduce infections.
susceptibility of the infecting bacterium is therefore
Vaccination can also be successfully used, especially
totally unpredictable. This is, however, also the case
against infections caused by C. perfringens, E. rhusio-
for staphylococci, where multiple resistant isolates are
pathiae, Mycoplasma, Lawsonia and virus infections.
recently more frequently observed. Thus, treatment hasto be based on knowledge at the individual farm.For Brachyspira it is especially worrying that resistance
Choice of antimicrobial agents for therapy
is emerging to the currently most active compoundtiamulin.
The licensing of veterinary medical products was until the
It is for some bacterial species to some extent still
last couple of decades to a large extent using limited
possible to predict the susceptibility. Thus, Mycoplasma
documentation for clinical efficacy. This has now changed
are still susceptible to tiamulin, and most A. pleuropneu-
and clinical trials are today required for licensing.
moniae, P. multocida and S. suis isolates are susceptible
However, there are very few independent studies that
to penicillins. Resistance to this group of antimicrobial
have compared the different available compounds for the
agents has emerged making it important that at least
same disease. Furthermore, information of clinical failure
national monitoring is performed to follow the trends in
due to the development of resistance is almost absent in
Some of the more recently approved antimicrobial
Antimicrobial susceptibility testing is practically useful
classes such as the cephalosporins and fluoroquinolones
in determining whether an antimicrobial should be used
are still active against a high frequency of isolates and can
to treat a condition, but should not be used as an absolute
therefore easily be preferred in many cases. However, as
result, only a guide. Susceptibility testing can be difficult
previously mentioned these antimicrobial classes are also
and requires the use of standard methods and use of
considered critically important for human health and
correct breakpoints for determining whether an isolate
their use in food animal production should be limited or
should be considered resistant or susceptible. Optimally,
avoided as far as possible. This makes it even more
difficult for the practising veterinarian to choose the most
among the target pathogen and high clinical efficacy
optimal treatment taking both the welfare of the animal
should be chosen for empiric treatment. Based on the
and the human health considerations into account. The
clinical experience and routine examination of clinical
most optimal way forward seems to be to ensure a more
samples and susceptibility testing this treatment might be
optimal production system with less dependence on
antimicrobial agents and to implement more continuous
However, choosing the right antimicrobial agent for
monitoring at the national, regional and down to the farm
treatment of infections in food animals is not only about
level to assist the veterinarian in choosing the most
the susceptibility of the animal pathogen. Using anti-
microbial agents for treatment of infections in foodanimals might also select for resistance that might be
transferred to humans and thereby cause human healthproblems (Aarestrup et al., 2008). Thus, whenever
This work was supported in part by grant 274-05-0117
initiating treatment of food animals the human health
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